‘My default setting now is to have some facial hair. As soon as I’m clean shaven I feel like Gareth.’

You can understand why Mackenzie Crook might want to distance himself – physically at least – from the role that first brought him to public notice: that of The Office’s Gareth Keenan, the corporate über-geek with a pudding-bowl haircut, an unnatural attachment to his stapler and a terrifying lack of cool.

Today, between rehearsals for new play The Aliens at Bush Theatre, Crook is sporting earrings and a Van Dyke beard that give him a rather raffish, piratical look. ‘It’s probably a conscious attempt to look as different from him as I can,’ he smiles.

If Crook’s most high-profile work since The Office has been playing Ragetti in the Pirates Of The Caribbean movie franchise – it earned him his own plastic action figure, after all – it’s his new-found devotion to the stage that’s brought him the greatest critical plaudits. But Crook didn’t go to drama school.

‘That’s probably one of the reasons I avoided theatre for so long,’ he reflects with characteristic tentativeness. ‘I didn’t feel I was qualified or that I had the right, almost. I don’t have that depth of knowledge about plays and playwrights. I don’t know my Shakespeare… I felt like a bit of a pretender because I didn’t have that training.’

Highly acclaimed performances in Chekhov’s The Seagull and, most recently, Jez Butterworth’s Jerusalem, however, have helped him to overcome any residual anxiety about his entitlement to act on stage.

Being involved in the hugely successful Jerusalem was a revelation. ‘The reaction it got was euphoric, a really joyous reception.’ Crook thinks the play may have struck such a deep chord with audiences because of its challenge to ‘corporate management society, which is making everything bland and paving over everything so that nobody trips up’. That it avoided preachiness also helped, he thinks. ‘It was done in an incredibly magical and entertaining way.’

He’s clearly attracted to theatre’s strong work ethic. ‘With television and film, the time when you’re in front of the camera can be counted in minutes, whereas with theatre you have full working days during the rehearsal period and when you’re performing you put so much into it, it’s the equivalent of a full working day because you’re exhausted after it. It’s a great feeling for someone who’s sat around doing nothing for a good few years.’

As an interviewee, Crook is thoughtful and obliging but he also has a mildly tormented air that suggests he’s not entirely at home with the media process.

‘I accept it as part of the job,’ he says. ‘If you want to be a successful actor you’re going to be well known in public. But I do feel uncomfortable thinking of myself as a celebrity. I shy away from panel shows and things like that. I think actors need to keep themselves a little bit private because the more people see of the real person, the less likely they are to believe you in a role.’

Despite regularly rubbing shoulders with the likes of Johnny Depp (‘I get on really well with him but I don’t have his mobile phone number and we don’t, you know, go out for a drink’) and having recently added a project with Steven Spielberg to his increasingly impressive CV (‘It’s not a massive role but I would have made the tea just to be involved’), Crook remains resolutely unstarry.

Indeed, it’s a mark of Crook’s lack of celebrity pretension that he is currently appearing in a play on the London fringe. ‘It’s great to do something in a very small space,’ he enthuses about his role in US dramatist Annie Baker’s three-hander The Aliens at the Bush. ‘On a big stage in the West End, you don’t have to look at the audience. You can’t see them – they’re out of focus or in the dark. Whereas at the Bush, they’re right there and you can see the audience turning their heads to look at you when you’re speaking and turning away when somebody else is speaking.’

Crook describes his character – thirty-something slacker Jasper – as ‘very serious, morose. I very often get offered wacky or quirky parts and he’s not like that’. It’s not typecasting, then. The role is a departure for him in another respect too. ‘All three characters are pretty low status,’ he explains. ‘They’re just hanging out at the back of a coffee shop. But I’m the one that the others look up to. I don’t often play that kind of role. Within the three, I am the alpha male!’